Mass evacuations are to take place in the Florida Keys and the Caribbean after a hurricane the size of France became the most powerful storm ever recorded in the Atlantic Ocean with 185mph winds. 

America's National Hurricane Center said Irma had strengthened into a dangerous Category 5 storm as it made its first landfall in islands of northeast Caribbean on a path that could take it to the United States - causing thousands to start panic buying and preparing to evacuate.

This morning, the eye of the 'potentially catastrophic' hurricane slammed into Barbuda just hours after officials warned people to seek protection from Irma's 'onslaught' in a statement that closed with: 'May God protect us all.'

Heavy rain and howling winds raked the neighboring island of Antigua, sending debris flying as people huddled in their homes or government shelters. The island of Anguilla is next in its path.

U.S. President Donald Trump declared emergencies in Florida, Puerto Rico and the U.S. Virgin Islands, and authorities in the Bahamas said they would evacuate the residents of six islands at the southern end of the island chain.  

Experts say Irma is now so powerful it is registering on devices designed to detect earthquakes. Scientists picked up the background noise of winds causing trees to move and crashing ocean waves on their earthquake-detecting seismometers. 

It is expected to become the second powerful storm to thrash the U.S. mainland in as many weeks after devastating Hurricane Harvey. 

In addition to Irma, Tropical Storm Jose has now formed behind it in the open Atlantic far from land. Jose is the 10th tropical storm of the season. It has maximum sustained winds of 40 mph and is about 1505 miles east of the Lesser Antilles.  Four other storms have had winds that strong in the overall Atlantic region, but they have been in the Caribbean Sea or the Gulf of Mexico where the usually warmer waters fuel tropical cyclones. Experts say Irma's strength is a result of unusually warm water for that part of the Atlantic. 

The center said there was a growing possibility that the storm's effects could be felt in Florida later this week and over the weekend, though it was still too early to be sure of its future track: 'Everyone in hurricane-prone areas should ensure that they have their hurricane plan in place.'

So far, a state of emergency has been called in the state and a mandatory evacuation is under way in the Florida Keys.  Schools there are also cancelled until further notice. 

Governor Rick Scott activated 100 members of the Florida National Guard to be deployed across the state, and 7,000 more National Guard members were to report for duty on Friday when the storm could be approaching. 

Pictures showed people rushing to stock up on drinking water, protective wooden boards for their windows and other supplies.

Meanwhile, tourists have had their Caribbean holiday plans thrown into chaos as Hurricane Irma forces airlines to ground or divert flights.

Antigua airport will be closed on Wednesday and San Juan airport, the busiest in Puerto Rico, has cancelled about 40 per cent of its flights in response to the hurricane.

British Airways sent an empty aircraft to the region to bring customers back early - the full flight of 326 passengers touched down in the UK on Tuesday evening.

It also cancelled a flight from the UK heading to Antigua and then on to Tobago.